Single-phase connection of a person to an electrical network. Analysis of the risk of electric shock in various electrical networks Danger of connecting a person to an electrical network

The defeat of a person by electric current as a result of electric impact, i.e., the passage of current through a person, is a consequence of his touching 2 points of the electrical circuit, between which there is some voltage. The danger of such a touch is assessed, as is known, by the current passing through the human body or by the voltage under which he finds himself. It should be noted that the touch voltage depends on a number of factors: the circuit for connecting a person to the electrical circuit, the voltage of the network, the circuit of the network itself, the mode of its neutral, the degree of isolation of the live parts from the ground, as well as the capacitance of the live parts relative to the ground, etc.

Consequently, the above danger is not unambiguous: in one case, the inclusion of a person in an electrical circuit will be accompanied by the passage of small currents through it and will not be very dangerous, in other cases the currents can reach significant values \u200b\u200bthat can lead to death. This article examines the dependence of the danger of including a person in an electrical circuit, that is, the values \u200b\u200bof the touch voltage and current flowing through a person, on the listed factors.

This dependence must be known when assessing a particular network according to safety regulations, choosing and calculating appropriate protection measures, in particular grounding, neutralization, protective shutdown, network isolation monitoring devices, etc.

In this case, in all cases, except for those specifically mentioned, we will assume that the resistance of the foundation on which the person stands (ground, floor, etc.), as well as the resistance of his shoes, are insignificant and therefore they can be taken equal to zero.

So, the most typical schemes for connecting a person to an electrical circuit when accidentally touching live conductors are:

1. Connection between two phase conductors of the circuit,

2. Connection between phase and earth.

Of course, in the second option, it is assumed that the considered network is electrically connected to the ground due to, for example, grounding the neutral of the current source or due to poor insulation of the wires relative to the ground, or due to the presence of a large capacitance between them.

Two-phase touch is considered the most dangerous, since in this case a line voltage of 380 volts is applied to the human body, and the current passing through the body does not depend on the network circuit and its neutral mode.

Two-phase touching occurs very rarely and is mainly associated with working under voltage:

On electrical panels, assemblies and overhead lines;

When using faulty personal protective equipment;

On equipment with unshielded live parts, etc.

Single-phase contact is usually considered less dangerous, since the current passing through the person in this case is limited by the influence of a number of factors. But it happens in practice much more often than biphasic. Therefore, the topic of this article is to analyze only cases of single-phase contact in the networks under consideration.

In case of electric shock to a person it is necessary to take measures to free the victim from the current and immediately begin to provide him with first aid.

Free a person from the action of the current it is necessary as soon as possible, but precautions must be taken. If the victim is at a height, measures should be taken to prevent him from falling.

Touching an energized person, it is dangerous, and when conducting rescue operations, it is necessary to strictly observe certain precautions against possible electric shock to persons carrying out these operations.

The easiest way to release the victim from the current is disconnection of an electrical installation or that part of it that a person touches... When the unit is turned off, the electric light may go out, therefore, in the absence of daylight, it is necessary to have another light source ready - a lantern, a candle, etc.

After releasing the victim from the currentit is necessary to establish the degree of damage and, in accordance with the condition of the victim, provide him with medical assistance. If the victim has not lost consciousness, it is necessary to provide him with rest, and in the presence of injuries or injuries (bruises, fractures, dislocations, burns, etc.), he must be given first aid before the arrival of a doctor or taken to the nearest medical institution.

If the victim has lost consciousness, but breathing is preserved, it is necessary to lay him evenly and comfortably on a soft bedding - a blanket, clothes, etc., unfasten the collar, belt, remove embarrassing clothing, cleanse the mouth of blood, mucus, provide fresh air, Allow to smell ammonia, sprinkle with water, grind and warm the body.

In the absence of signs of life (in case of clinical death, there is no breathing and pulse, the pupils of the eyes are dilated due to oxygen starvation of the cerebral cortex) or in case of intermittent breathing, the victim should be quickly released from the clothing that restrains breathing, cleansed the mouth and do artificial respiration and heart massage.

Such diseases that aggravate the outcome of electrical trauma include: increased thyroid function, many diseases of the nervous system, angina pectoris. The influence of alcoholic intoxication should be especially noted. In addition to the fact that a person in a state of alcoholic intoxication often makes mistakes and receives electrical injuries, due to alcohol intoxication, the central nervous system loses its regulatory role in controlling breathing and blood circulation, which significantly aggravates the outcome of the lesion.

Inclusion of a person in an electric current circuit

Reasons for inclusion. A person is included in an electric current circuit with direct contact of the body with a live part of an electrical installation that is energized. This usually happens due to negligence or as a result of erroneous human actions, as well as due to a malfunction of electrical installations and technical means of protection. Such cases, for example, include the following:

Touching live parts that are energized, on the assumption that they are de-energized;

Touching during repair, cleaning or inspection to previously de-energized live parts, but to which an unauthorized person has mistakenly energized or spontaneously turned on a faulty starting device;

Touching metal parts of electrical installations, which are usually not energized, but were energized relative to earth due to damage to electrical insulation or other reasons (short circuit to the case);

The appearance of a step voltage on the surface of a conductive base (floor), along which a person passes; and etc.

Connection diagrams. A person can connect to an electric current circuit by touching one phase of an electrical installation under voltage, simultaneously to two phases or to a zero protective conductor and a phase. Contact with a zero protective conductor is safe (Fig. 2, a, I), other cases entail serious consequences.

Fig. 2. Diagrams of the paths of passage of electric current through the human body: a - touching the wires; b - the occurrence of touch voltage; c - The emergence of a step voltage; I-touch to the neutral wire; II - touching the phase wire; III - touching the phase and neutral wires; IV - touching the phase wires; 0 - neutral wire; 1, 2, 3 - phase wires; 4 - neutral point; 5- single earthing (electrode); A, B, C - electrical installations

Single-phase (single-pole) touch (Fig. 2, a, II and III) occurs most often when replacing lamps and maintaining lamps, changing fuses and maintaining electrical installations, etc. In a system with a neutral grounded, a person will be under the phase voltage Uph (in V), which is less than the linear Ul:

Accordingly, the value of the phase current passing through the human body will also be less. If at the same time a person is reliably isolated from the ground (shod in dielectric galoshes, the floor is dry and non-conductive), then a single-phase touch is not dangerous.

Two-phase (two-pole touching) touching is more dangerous, because a person falls under line voltage (Fig. 2, a, IV). Even with a voltage of 127 V and a calculated value of the human body resistance of 1000 Ohm, the current in the circuit will turn out to be fatal (127 mA). With two-phase contact, the risk of injury will not decrease even if the person is reliably isolated from the ground (floor).

Two-phase contact is rare, usually during live work, which is strictly prohibited.

If the insulation of live parts is damaged and shorted to the body of electrical equipment, a significant potential may appear. In this case, a person who touches the body of the electrical installation (Fig. 2, b) will be under the touch voltage Uп (in V)

where Ich is the value of the current passing through a person along the "hand-foot" path, A; Rh - resistance of the human body, Ohm.

Touch voltage is the potential difference between two points of the electrical circuit, which are simultaneously touched by a person, or the voltage drop in the resistance of the human body.

The touch voltage will increase as the distance between the electrical installation and the ground electrode increases, reaching a maximum at a distance of 20 m or more. When the phase wire falls on the earth's surface, a current spreading zone appears (Fig. 2, c).

A person passing through this zone will be under a step voltage (potential difference) between two points of the current circuit located at a step distance (0.8 m) from one another. The largest step voltage will be near the closing point and, gradually decreasing, at a distance of 20 m, it will drop to zero.

Do not approach the fallen wire closer than 6-8 m. If necessary, the approach should be de-energized or put on dielectric galoshes (boots).

Psycho-emotional alertness - the "factor of attention" when working with electric shock

The formation of psycho-emotional alertness in workers, the "factor of attention" when working with electric current is the most important condition for personal prevention of electrical injuries. This factor is based on knowledge of the physiological effect of an electric current on the body when an injured person enters an electrical circuit.

In particular, the decisive role in many cases of lesions is played by the "attention factor", ie, in essence, the severity of the outcome of the lesion is largely determined by the state of the human nervous system at the time of the lesion.

It is necessary that the person was "collected", which allows you to expect any event during work that requires attention.

Such a statement is valid mainly in case of electric shock with a voltage of 220-300 V. At high voltages, a severe outcome most often occurs from arc burns. Here there is already reason to believe that the risk of burns increases almost linearly depending on the voltage value.

The attention factor undoubtedly causes the mobilization of the body's defense systems, enhances the blood circulation of the heart muscle, cerebral blood flow through the pituitary-adrenal system and makes them more resistant to external stimuli (electrical injury).

With the factor of attention, it is much more difficult to upset the biosystem of automatic regulation of the most important systems of the body (central nervous system, blood circulation, respiration).

However, it should be noted that the role of the attention factor is not yet sufficiently reflected in protective measures for electrical safety.

But there is confidence that new views on the electrical safety of living tissue, further study of the nature of the electrical activity of the human body makes it possible to reveal the biophysics of the mechanism of human damage, which will be taken into account in the development of measures to protect against the action of electric current.

Measures to ensure the safe operation of electrical equipment

Technical methods and means of protection, ensuring electrical safety, are indicated taking into account: power supply with electric power of rated voltage, type and frequency of current; neutral mode, type of performance; environmental conditions; the ability to remove voltage from live parts; the nature of the possible touch of a person to the elements of the current circuit.

All cases of electric shock to a person as a result of an electric shock are a consequence of touching at least two points of the electrical circuit, between which there is a potential difference. The danger of such a touch largely depends on the characteristics of the electrical network and the scheme for including a person into it. Having determined the amperage / hr passing through a person taking these factors into account, appropriate protective measures can be selected to reduce the risk of injury.

Two-phase inclusion of a person in the current circuit (Fig. 8.1, a). It happens quite rarely, but it is more dangerous compared to single-phase, since the highest voltage in this network is applied to the body - linear, and the current, A, passing through a person does not depend on the network circuit, its neutral mode and other factors, i.e. e.

I \u003d Ul / Rch \u003d √ 3Uph / Rch,

where Ul and Uf are linear and phase voltage, V; Rch is the resistance of the human body, Ohm (according to the Rules for Electrical Installations in the calculations, Rh is taken equal to 1000 Ohm).

Cases of two-phase contact can occur when working with electrical equipment without removing the voltage, for example, when replacing a blown fuse at the entrance to a building, using dielectric gloves with ruptured rubber, connecting a cable to unprotected clamps of a welding transformer, etc.

Single-phase inclusion. The current passing through a person is influenced by various factors, which reduces the risk of injury compared to two-phase contact.

Fig. 8.1. Schemes of the possible inclusion of a person in a three-phase current network:

a - two-phase touch; b - single-phase contact in a network with a grounded neutral; c - single-phase contact in a network with an isolated neutral

In a single-phase two-wire network, isolated from the ground, the current strength, A, passing through a person, when the insulation resistance of the wires is equal to the ground r1 \u003d r2 \u003d r, is determined by the formula

Ich \u003d U / (2Rh + r),

where U is the mains voltage, V; r - insulation resistance, Ohm.

In a three-wire network with an isolated neutral, at r1 \u003d r2 \u003d r3 \u003d r, the current will go from the point of contact through the human body, shoes, floor and imperfect insulation to other phases (Fig. 8.1, b). Then

Ich \u003d Uph / (Ro + r / 3),

where Ro is the total resistance, Ohm; RO \u003d Rh + Rop + Rp; Rob - shoe resistance, cm: for rubber footwear Rab ≥ 50 000 Ohm; Rn - floor resistance, Ohm: for a dry wooden floor, Rp \u003d 60,000 Ohm; d - insulation resistance of wires, Ohm (according to the PUE, it should be at least 0.5 megohm per phase of the network section with voltage up to 1000 V).

In three-phase four-wire networks, the current will go through the person, his shoes, the floor, the grounding of the neutral of the source and the neutral wire (Figure 8.1, c). The strength of the current, A, passing through a person,

Ich \u003d Uph (Rо + Rн),

where RH is the neutral grounding resistance, Ohm. Neglecting the resistance RH, we get:

At agricultural enterprises, they mainly use four-wire electrical networks with a dead-grounded neutral voltage of up to 1000 V. Their advantage is that through them two operating voltages can be obtained: linear Ul \u003d 380 V and phase Uph \u003d 220 V. Such networks are not presented. high requirements for the quality of insulation of wires and they are used with a large branching network. Somewhat less often, a three-wire network with an isolated neutral is used at voltages up to 1000V - it is safer if the insulation resistance of the wires is maintained at a high level.

Touch voltage. It occurs as a result of touching live electrical installations or metal parts of equipment.

Step voltage. This is the voltage Ush on the human body when the legs are positioned at the points of the current spreading field from the ground electrode or from the wire that fell to the ground, where the feet are located, when a person walks in the direction of the ground electrode (wire) or away from it (Fig. 8.2).

If one leg is at a distance x from the center of the ground electrode system, then the other is at a distance x + a, where a is the step length. Usually, in the calculations, a \u003d 0.8 m is taken.

The maximum voltage in this case arises at the point of the current short circuit to the ground, and with distance from it it decreases according to the law of hyperbola. It is considered that at a distance of 20 m from the point of short circuit, the earth potential is zero.

Step voltage, V,

Fig. 8.2. Step voltage generation circuit

Even with a small step voltage (50 ... 80 V), an involuntary convulsive contraction of the leg muscles can occur and, as a consequence, a person falls to the ground. At the same time, he simultaneously touches the ground with his hands and feet, the distance between which is greater than the length of the stride, so the acting stress increases. In addition, in this position of a person, a new pathway for the passage of current is formed, affecting the vital organs. This creates a real threat of fatal defeat. As the stride length decreases, the step voltage decreases. Therefore, in order to get out of the zone of action of the step voltage, one should move by jumping on one leg or on two closed legs, or in as short steps as possible (in the latter case, a voltage of no more than 40 V is considered permissible).

Analysis of electrical safety conditions

The analysis of electrical safety conditions consists in determining the magnitude of the current through the human body (I h) for a specific case.

Comparing the calculated values \u200b\u200bof the current through the human body with the value of the conditionally safe current (10mA), a conclusion is made about the danger of this case. If the value of the current through the human body exceeds the value of the conditionally safe current, the case is considered dangerous. If not, not dangerous. Since a person in most cases uses a network up to 1000V, and these networks, as a rule, have a short length, the capacitance of the phase wires relative to the ground can be neglected, considering that the insulation resistance of the wires (R from) relative to the ground is purely active.

You can determine the amount of current through the human body as follows:

I h \u003d U pr / R h

The complexity of the calculation lies in finding the touch voltage (U pr). To find this value, they resort to this technique: they determine the path of the current through the human body, from which they find the source of voltage and resistance through which the current flows.

The most typical are two connection schemes: between two wires and between one wire and ground.

With regard to AC networks, the first circuit is usually called two-phase connection, and the second single-phase.

9.1.1. Two-phase switching

A two-phase connection, as a rule, is more dangerous, since the highest voltage in this network is applied to the human body - linear, and therefore a large current will flow through the human body (Figure 9.1.).

Figure 9.1. Two-phase inclusion of a person in the network.

where, I h - current through the human body

U pr - touch voltage

For 380/220 network

Current dangerous to human life

9.1.2. Single-phase inclusion.

Single-phase switching occurs much more often, but it is less dangerous, because the voltage under which a person finds himself does not exceed the phase voltage. In addition, the neutral mode of the current source, the insulation resistance of the wires relative to the ground, the resistance of the floor on which the person stands, the resistance of the person's shoes and other factors also affect the value of the current through the human body.

9.1.2.1. Single-phase network.

Figure 9.3. Connection diagram

Figure 9.4. Equivalent circuit

The current through the human body can be found as:

From the expression, you can draw conclusions:

1. The greater the insulation resistance to ground, the less the danger of single-phase touching the wire.

2. The touch of a person to a wire with a high insulation resistance is more dangerous, because the tension of the touch will be greater.

9.1 1.2. Three-phase three-wire network with insulated neutral:

Consider two network modes:

a) Normal operation (insulation resistance is of great (normalized) value.

Figure 9.5. Single-phase connection to a 3-phase network

with isolated neutral

If the insulation resistances are equal R of1 \u003d R of2 \u003d R of3, the value of the current through the human body is determined by the expression

In such networks, the danger to a person touching the wire, during the normal state of the network, depends on the insulation resistance. The larger it is, the less danger. Therefore, it is very important in such networks to ensure a high insulation resistance and monitor its condition for timely detection and elimination of faults that have arisen.

According to PES, the insulation resistance of wires to ground in installations up to 1000V should not be less than 500k.

b) In emergency mode - short circuit of one of the phases to ground through a low short circuit resistance - R zm. (Figure 9.6.)

Figure 9.6 Network Emergency Mode

Usually R gm lies in the range from 50 to 200 Ohm.

The current through the human body, as in the normal mode, will also flow through the insulation resistance of the wires relative to the ground, but its value will be much less than the current flowing through the small circuit resistance. Therefore, the magnitude of the current flowing through the insulation resistance can be neglected and it can be assumed that the current flows only through the circuit resistance and the human body.

It is very dangerous.

9.1.2.3. Three-phase three-wire network with a dead-earthed neutral:

Solidly grounded is the neutral of a transformer or generator connected to a grounding device directly or through a low resistance (for example, a current transformer).

a) Normal operation

Figure 9.7.

The neutral grounding resistance R о is standardized depending on the maximum mains voltage.

With U l \u003d 660V, R about \u003d 2 Ohm, with U l \u003d 380V, R about \u003d 4 Ohm, with U l \u003d 220V, R about \u003d 8 Ohm

The current flowing through the human body and the insulation resistance of the wires can be neglected, compared to the current flowing through the human body and the low resistance of the neutral grounding. The magnitude of this current is determined from the expression:

It can be seen from the expression that in a network with a dead-grounded neutral during the period of normal operation of the network, touching one of the wires is more dangerous than touching a wire of a normally operating network with an insulated net.

b) In emergency operation - when one of the phases of the network is shorted to ground through a low resistance R zm (Figure 9.8.).

Figure 9.8.

If we analyze this case, then we can draw the following conclusions:

2. If we take R about equal to 0, then the person will be under phase voltage.

In real conditions, R zm and R about are always greater than zero, therefore, a person, touching the wire in the emergency mode of the network, gets under voltage less than linear, but more than phase voltage.

Table of contents of the book Next page \u003e\u003e

§ 3. Danger of electric shock to persons.

Scheme of a single-phase connection of a person to a three-phase current network with a grounded neutral.

The defeat of a person by current occurs when an electrical circuit is closed through the human body. This occurs when a person touches at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in a circuit can occur according to several schemes: between the wire and the ground, called single-phase switching; between two wires - two-phase connection. These circuits are most typical for three-phase AC networks. It is also possible to connect between two wires and ground at the same time; between two points on the earth with different potentials, etc.

Single-phase inclusion of a person into the network represents direct human contact with parts of an electrical installation or equipment that are normally or accidentally energized. In this case, the degree of danger of injury will be different depending on whether the electrical network has a grounded or isolated neutral, as well as depending on the quality of the insulation of the network wires, its length, operating mode and a number of other parameters.

With a single-phase connection to a network with a grounded neutral, a person falls under a phase voltage that is 1.73 times less than a linear voltage, and is exposed to a current, the value of which is determined by the value of the phase voltage of the installation and the resistance of the human body (Fig. 69). An additional protective effect is provided by the insulation of the floor on which the person is standing and the shoes.

Fig. 69. Scheme of a single-phase connection of a person to a three-phase current network with a grounded neutral

Thus, in a four-wire three-phase network with a grounded neutral, the current circuit passing through a person includes the resistance of his body, as well as the resistance of the floor, shoes and grounding of the neutral of the current source (transformer, etc.). In this case, the current value

where U l - line voltage, V; R t is the resistance of the human body, Ohm; R p is the resistance of the floor on which the person is located, Ohm; R about - resistance of human shoes, Ohm; R 0 - neutral grounding resistance, Ohm.

As an example, let us consider two cases of single-phase connection of a person into a three-phase four-wire electrical network with a grounded neutral at U l \u003d 380 V.

A case with unfavorable conditions... A person who has touched one phase is on damp ground or a conductive (metal) floor, his shoes are damp or have metal nails. In accordance with this, we take the resistances: human body R t \u003d 1000 Ohm, soil or floor R p \u003d 0; shoe R about \u003d 0.

Neutral grounding resistance R 0 \u003d 4 Ohm is not taken into account in view of its insignificant value. A current will pass through the human body

which is life-threatening.

A case with favorable conditions... A person is on a dry wooden floor with a resistance of R p \u003d 60,000 ohms, has dry non-conductive (rubber) shoes on his feet with a resistance of R p \u003d 50,000 ohms. Then a current will pass through the human body

which is long-term permissible for humans.

In addition, dry floors and rubber shoes have significantly higher resistance compared to the values \u200b\u200bused for the calculation.

These examples show the great importance of the insulating properties of the floor and footwear to ensure the safety of persons working in conditions of possible contact with electric shock.